Method and apparatus for forming articles from a tubular blank

ABSTRACT

A HYDROSTATIC METHOD AND APPARATUS FOR FORMING A TUBULAR BLANK INTO IRREGULAR SHAPES AND CONFIGURATIONS THROUGH THE APPLICATION OF OUTWARDLY DIRECTED PRESSURE FORCES APPLIED AGAINST THE INTERIOR WALLS OF THE TUBULAR BLANK. THE APPARATUS IS PROVIDED WITH A FORMING THE WHICH CLOSES TO PROVIDE A LONGITUDINAL PASSAGE FOR RECEIVING AND FORMING THE TUBULAR BLANK. A PAIR OF PLUGS INSERTED WITHIN OPPOSITE ENDS OF THE TUBULAR BLANK ARE PROVIDED WITH PERIPHERAL SURFACES ADAPTED TO FORM FLUID SEALING ENGAGEMENT BETWEEN THE PLUGS AND THE ASSOCIATED INTERIOR WALLS OF THE TUBULAR BLANK. IN THE PREFERRED EMBODIMENT THE PERIPHERAL SURFACE OF EACH PLUG HAS AN ELASTOMERIC MATERIAL DISPOSED THEREIN ADAPTED TO FORM AND MAINTAIN FLUID SEALING ENGAGEMENT IN RESPONSE TO THE PRESSURE APPLIED WITHIN THE TUBULAR BLANK. PISTONS ASSOCIATED WITH EACH PLUG ARE RESPONSIVE TO THE PRESSURE WITHIN THE TUBULAR BLANK FOR URGING THE PLUGS INWARDLY INTO THE DIE PASSAGE TO PREVENT SEPARATION OF THE SEAL FROM THE BLANK AS SAME IS SHORTENED DURING THE SHAPING PROCESS.

Dec. 7, 1971 w, DE GA|N 3,625,040

METHOD AND APPARATUS FOR FORMING ARTICLES FROM A TUBULAR BLANK Filed Aug. 6, 1969 2 Sheets-Sheet 1 I I I I I I III[III/III]IIIIIIIIIlI/IIIIII INVENTOR WILLIAM J. DEGAIN ATTORNEYS Dem 7, 1971 w. DE GAIN 3,625,040

METHOD AND APPARATUS FOR FORMING ARTICLES FROM A TUBULAR BLANK Filed Aug 6, 1969 2 Sheets-Sheet 2 FIG.9

mvanrron WILLIAM J. DEGAIN Bv am ATTORNEYS United States Patent 3,625,040 METHOD AND APPARATUS FOR FORMING ARTHCLES FROM A TUBULAR BLANK William J. De Gain, Warren, Mich, assignor to Koppy Tool Corporation, Ferndale, Mich. Filed Aug. 6, 1969, Ser. No. 847,837 Int. Cl. B21d 26/04 US. Cl. 7259 5 Claims ABSTRACT OF THE DISCLOSURE A hydrostatic method and apparatus for forming a tubular blank into irregular shapes and configurations through the application of outwardly directed pressure forces applied against the interior walls of the tubular blank. The apparatus is provided with a forming die which closes to provide a longitudinal passage for receiving and forming the tubular blank. A pair of plugs inserted within opposite ends of the tubular blank are provided with peripheral surfaces adapted to form fluid sealing engagement between the plugs and the associated interior walls of the tubular blank. In the preferred embodiment the peripheral surface of each plug has an elastomeric material disposed therein adapted to form and maintain fluid sealing engagement in response to the pressure applied within the tubular blank. Pistons associated with each plug are responsive to the pressure within the tubular blank for urging the plugs inwardly into the die passage to prevent separation of the seal from the blank as same is shortened during the shaping process.

BACKGROUND OF THE INVENTION (I) Field of the invention This invention relates to the art of hydrostatic metal forming and particularly to the forming of tubular parts into products having irregular and complicated shapes and configurations.

(II) Description of the prior art Numerous hydrostatic or hydroforming methods have been proposed and employed for forming tubular blanks into parts having irregular shapes and configurations by the application of pressure. Usually such methods employ dies into which a tubular blank is inserted and its opposite ends are hydraulically sealed. Hydraulic pressure is then generated within the tubular blank and the outwardly directed force generated therein is exerted against the interior walls of the tubular blank until the desired shape is obtained. Since the magnitude of the pressure required in such applications can be substantial it is necessary to provide a sealed hydraulic connection at the ends of the tubular blank which cannot only sustain such high pressures but which is axially shiftable inwardly within the die, as the length of the tubular blank is shortened during the shaping process.

In prior art devices the hydraulic sealing of the opposite ends of the tubular blank is generally achieved by firmly pressing annular members against the open ends of the blank so as to insure fluid tightness, or O-rings are utilized in the conventional manner in combination with plugs inserted within the open ends of the tubular blank for obtaining the desired sealing. Such conventional designs ordinarily operate satisfactorily in a range of 15000 to 20000 p.s.i. but are not well suited for high pressure operation up to about 50000 p.s.i. as proposed herein, being pressures generally below the extremely high pressures of explosive, electro-arc and the like types of forming processes.

SUMMARY OF THE INVENTION The preferred embodiment of the present invention, which will be described in more detail hereinafter, comprises an apparatus for forming tubular blanks into various shapes and configurations through the application of a pressure force applied against the interior walls of the tubular blank. A split forming die having opposing halves movable to and from one another to permit easy feeding and ejection of the tubular blank, forms when closed a longitudinal passage in which the tubular blank is shaped. The forming apparatus is submerged in a fluid during the shaping of the blank so as to prevent the possibility of an adiabatic explosion due to air entrapment and serves as a safety factor to dissipate energy in the event of a failure. A pair of plugs which are of a shape complementary to the shape of the opposite ends of the tubular blank are adapted to be received therein to a predetermined distance. Each of the plugs has an outer diameter which exceeds the inner diameter of the tubular blank by a predetermined amount creating an interference between the two parts to provide a temporary seal. Thus, as the plugs are pressed inwardly into the tubular blank the outer ends of the tubular blank are slightly expanded. Each of the plugs is provided with a peripheral groove around the portion which engages the inner wall at the ends of the tubular blank and in which an elastomeric seal is disposed, and means are provided to expand the seal to create and maintain a fluid seal between the plug and the inner wall when the interior of the tubular btlank is subjected to a high pressure. Since the ends of the blank may, and probably will not, fit closely within the die, which has to be large enough to accommodate varying material wall thicknesses, expansion of the blank during forming causes the temporary seal of the interference fit between blank and plugs to be lost, hence the necessity for the expanding seal.

The outer ends of the plugs form one Wall each of a pair of expansible pressure chambers, which are communicated to the interior of the tubular blank by axial passageways extending through each plug. These passageways are connected with the inner sides of the seal by means of a plurality of radially etxending passageways.

At least one of the plugs has a plunger extending through its longitudinal passageway and into the interior of the tubular blank. When the tubular blank is ready to be formed, it is completely filled with the fluid within which it and the apparatus are submerged, and sealed at opposite ends by means of the plugs. The formation of the tubular blank to the desired shape and configuration is obtained by moving the plunger through its associated plug into the interior of the tubular blank, whereupon the displaced fluid causes a substantial pressure increase therein. As the pressure increases, it creates a force against the interior wall of the tubular blank urging the same outwardly into engagement with the die, forcing the blank to assume the shape of the interior walls of the die. In addition, this pressure increase within the tubular blank is communicated by the axial and radial passages within the plugs to the undersides of the seals urging same radially outwardly into a tight filled sealing engagement between the plugs and the inner wall of the tubular blank to prevent the leakage of fluid therebetween.

The longitudinal passageways associated with each plug communicate the pressure within the interior of the tubular blank to the outer end walls of each plug and the associated expansible pressure chambers. The force generated against the outer sides of the plugs tend to urge the plugs inwardly and provides a two-fold function. The first is to cause the plugs to follow the tube as the same is shortened due to the formation process to prevent the blank from drawing away from the seals; and, second, to help urge the tubular blank to move toward the forming areas of the die.

Upon formation of the tubular blank to the desired shape the plugs are withdrawn, the die is opened, and an ejector of suitable means removes the tubular part from the die.

In a second embodiment of the present invention, each of the plugs has an outer diameter which exceeds the inner diameter of the tubular blank by a predetermined uniform amount along the inner portion of the inserted I section. That portion of the inserted section of each plug associated with the ends of the tubular blank interior wall is provided with an outwardly tapered shape and forms an annular flared section at the opposite ends of the tubular blank upon insertion therein. The plug tapered section and a mating tapered annular section formed on opposite ends of the die together from a step which locks the opposite ends of the tubular blank against axial movement and provides a fluid tight seal DESCRIPTION OF THE DRAWINGS The description refers to the accompanying drawings in which like reference characters refer to like parts throughout the several views and in which:

:FIG. 1 is a diagrammatic sectional view of an apparatus incorporating a preferred embodiment of the present invention and taken along line 11 of FIG. 5;

FIG. 2 is a view of the apparatus of FIG. 1 illustrating an intermediate stage in the formation of the tubular blank;

FIG. 3 is a view of the apparatus of FIGS. 1 and 2 illustrating another intermediate stage in the formation of the tubular blank;

FIG. 4 is a view of the apparatus of FIGS. 1, 2 and 3 illustrating a final stage in the forming operation;

FIG. 5 is a diagrammatic sectional view of the apparatus illustrated in FIG, 1 taken along line 5-5 thereof;

FIG. 6 is an enlarged fragmentary view illustrating in an exaggerated form the interference between one of the plugs and an end of the tubular blank;

FIG. 7 is an enlarged fragmentary view illustrating in an exaggerated form the engagement of the plug with an end of the tubular blank;

FIG. 8 is a fragmentary view similar to FIG. 7 showing the position of the seal during the initial stage of the shaping process;

FIG. 9 is a fragmentary view similar to FIG. 8 showing the position of the seal during the final stage of the shaping process; and

-FIG. 10 is a fragmentary view of another embodiment of the present invention in which there is illustrated in an exaggerated form the engagement of a plug with one end of the tubular blank.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, and especially FIGS. 1 and 5, there is shown a preferred tubular blank forming apparatus 10 disposed within a tank which is schematically illustrated and generally indicated by the numeral 12. The tank 12 is filled with a liquid 14 to a predetermined level 14a, submerging the entire apparatus as indicated in FIG. 5, The liquid 14- which may well be water, however it should be noted that other liquids which have a low coefficient of compressability may be preferred.

A split forming die 16 has opposing halves indicated by the numerals 18 and 20. The die half 18 comprises ,a generally rectangularly shaped block of metal, such as steel, having a substantially semi-cylindrically shaped recess 22 extending across the inner face thereof, and in which a plurality of grooves or impressions 24 are provided for forming the desired shape of a tubular blank 25. The die half 20 is likewise a rectangularly shaped block having a substantially semi-cylindrically shaped recess 26 and grooves 28 formed along its inner face. The recesses 22 and 26 of the die halves are adapted to face each other in order to provide a substantially cylindrical bore 30 through the length of the composite die. A plurality of radial passages 31 extend through the die halves 18 and 20 and are provided for venting the cylindrical hole '30 during the forming stages as will be described in more detail hereinafter. Although a cylindrically shaped forming surface is illustrated, it is to be understood that the tubular blank may be formed into any desired shape such as an oval, a square, or the like and may have other configurations than the grooved one described.

For maximum facility in operation, the die halves 18 and 20 are mounted on the inner ends of a pair of horizontally moving hydraulic rams, not shown. The hydraulic rams are exteriorly actuated by means customary to the art to close and hold the split die structure together, when expansion operations are conducted therein, with suflicient force so that the die halves 18 and 20' will not separate.

The tubular blanks 25 are fed into the apparatus 10 by any suitable feeding means (not shown) positioned above the split die halves 18 and 20, while a part ejector 29 (FIG. 5) positioned below the split die halves is adapted to move vertically upward between the split die halves 18 and 20 to eject the formed tubular blank 25 after it is formed and the dies are separated.

At opposite ends and aligned with the cylindrical bore 30 there are provided a pair of axially aligned plugs 32 and 34, each of which is slidably disposed within mounting blocks 36 and 38 respectively. The mounting blocks 36, 38 and thus the plugs 32, 34' are exteriorly actuated by any suitable well known means, such as hydraulic rams (not shown), to move them inwardly into the die cylindrical hole 30 at some controlled speed and force. The plugs 32, 34 are shaped substantially complementary to the shape of the ends of the tubular blank 25 and are illustrated as being of a generally cylindrical shape having a plurality of step sections 40, 42 and 44 formed thereon. The step section 44 is of the larger diameter and is slidably disposed within a recess 46 formed within each of the end blocks 36, 38. The outer surface 48 of each plug cooperates with the inner portion of the respective recess 46 to form an expansible pressure chamber 50 Within each of the blocks 36, 38. A suitable sealing means such as an elastomeric material or the like is provided at 52 within a peripheral groove 54 around the step portion 44 so as to maintain a fluid sealing sliding engagement between the plug and its associated recess. The step section 40 is adapted to be inserted within the interior of the tubular blank 25 until the opposite ends 56 of the tubular blank abut against shoulders 58 formed between the step portions 40, 4-2. The diameter of the step section 4 2 is such as to insure that the shoulder 58 will fully cover the end edge of the blank.

As can be seen in FIG. 6, the outer diameter of the step section 40 is so sized as to be slightly larger than the inner diameter of the tubular blank 25, in order to create a slight interference between the two engaging parts so as to provide a temporary mechanical seal when the parts are engaged in the position of FIG. 3. The extreme inner ends 80 of the plugs are preferably slightly tapered or beveled to initially permit easy access to the interior of the tubular blank 25.

A peripheral groove 60 formed around the step section 40 has a flexible sealing element disposed therein, such as an elastomeric material or the like and is shown, for purposes of illustration only, as an O-ring seal 62. The seal 62 is of such a size that it normally seats in the bottom of the groove 60, its cross-sectional width being approximately equal to the axial width of the peripheral groove 60' while its outer diameter in the non-expanded state is preferably slightly less than the outer diameter of the step section 40 so as to prevent interference between the seal 62 and the end of the tubular blank 25 when the same is initially forced over the plug 32, 34, thus avoiding any problem of scrubbing, pinching or chafing of the seal 62 by the blank and hence ensuring a long life for the seal 62.

Each of the plugs 32, 34 is provided with an axial bore 64 extending between opposite surfaces to connect the pressure chambers 50' with the interior of the tubular blank 25 for a purpose which will be described hereinafter. The inner surface of the peripheral groove 60' and thus the lower portion of the seal 62 is connected to the interior of the tubular blank by means of a plurality of radially inwardly extending passages 66 and the axial passageway 64. The pressure chamber 50 formed within the mounting block 36 is fluidly connected to the liquid 14 within the tank 12 by means of a passageway 70 extending axially therethrough. A check valve 72 disposed therein permits fluid flow unidirectionally from the tank 12 to the pressure chamber 50 to compensate for fluid displaced from the die through the passages 31 when the blank is expanded.

The mounting block 38 is provided with an axial bore 74 in which a plunger 76 is mounted for reciprocal movement. Suitable sealing means are provided at 78 to prevent the leakage of fluid from the interior of the blank through the bore 74. The plunger 76 extends axially through the plug bore 64- and is adapted to be moved into the interior of the tubular blank 25 to displace the fluid therein and create a substantial pressure rise wherein an outwardly directed pressure force is applied against the interior walls of the tubular blank 25 forcing the same to expand outwardly into the grooves 24 and 28 formed within the die halves 18, 20 and which will be described in more detail hereinafter.

In operation, the tubular blank 25 is inserted downwardly into position between the die halves 18, 20 which are then brought together as illustrated in FIG. 2. The plugs 32, 34 and their associated blocks 36, 38 are next moved axially toward the tubular blank 25 until the step portions 40 of the plugs are forced into the outer ends of the tubular blank 25, expanding them as shown to form the heretofor described temporary mechanical seal indicated in FIG. 7, and the ends 56 of the tubular blank now abut the plug shoulders 58. In this position, the inner surfaces 82 of the blocks 36, 38 are in abutment with the outer ends 84 of the die halves 18, 20. Vents 86 provided between the juncture of the block surfaces 82 and the'die ends 84 provide proper venting of the fluid trapped between the adjacent components. Upon inspection of FIG. 7, it will be noted that the seal 62 disposed within the peripheral groove 60 in each plug is inwardly spaced from and thus will not be damaged by the engaged portion 88 of the tubular blank 25.

Since the walls of the tubular blank 25 may vary in thickness from part to part, the diameter of the cylindrical bore 30 of the closed die must be sized to accommodate the largest sized tubular blank. Thus when the die halves 18, 20 are brought together as illustrated in FIG. 2, a clearance space 89 is formed between the outer periphery 6 of the tubular blank 25 and the surface of the cylindrical bore 30 With the plugs entered into and engaged with the outer ends of the tubular blank 25 and the block surfaces 82 engaged with the outer ends 84 of the die halves 18, 20, the device is in a position to commence the forming of the tubular blank to the desired shape and configuration.

As the plunger 76 is moved leftwardly (as viewed in the drawings) into the interior of the tubular blank 25 and displaces the fluid therein, the pressure within the interior of the tubular blank 25 will be substantially increased. The pressure generated within the tubular blank 25 exerts an outwardly directed force against the interior walls 90 of the tubular blank 25 forcing the metal surfaces 92 to expand outwardly and take the shape of the forming die grooves 24 and 28. Simultaneously with the movement of the plunger 76 into the interior of the tubular blank 25, the pressure therein is communicated to the underside of the seal 62 within the peripheral groove 60 by means of the plug passageways 64, 66. As this pressure is increased, the force generated thereby is exerted against the seal 62 pushing it radially outward against the engaged portion 88 of the tubular blank interior Wall 90, thereby creating a tight fluid sealing relationship between the plug step section 40 and the tubular blank engaged portion 88 and which is illustrated in an enlarged sectional view in FIG. 8.

As the plunger is moved inwardly into the tubular blank interior and the pressure therein is exerted against interior walls 90 of the tubular blank 25 to cause the same to expand, the axial length of the tubular blank 25 is shortened while the portions 88 of the tubular blank which engage the plug step 40 are expanded outwardly into the clearance space 89 into abutment with the die halves 18, 20, creating a clearance space 93 between the plug steps 40 and the tubular wall portions 88 as illustrated in FIGS. 4 and 9. In order to maintain the fluid sealing relationship created by means of the seal 62, fluid is communicated to the pressure chambers 50 in each of the blocks by means of the axial passageways 64. Since the area of the surface 48 exposed to fluid pressure within the chamber 50 is greater than the area exposed to pressure at the inner side of the plugs 32, 34 within the interior of the tubular blank 25, the resultant forces exerted on the plugs 32, 34 are such as to urge the plugs inwardly towards the interior of the tubular blank 25. Thus, as the tubular blank 25 is shortened by the radially outward expansion thereof, and as the tubular blank portions 88 separate from the plug steps 40, the plugs follow the tubular blank and thus maintain the pressure responsive seal 62 in a fluid sealing engagement with the plug step section 40 and the engaged portion 88 of the tubular blank 5 (FIG. 9). At the same time the inwardly moving plugs assist the tubular blank in its axial inwardly movement and thus in its outward expansion.

As the tubular blank 25 expands outwardly into the forming grooves 24 and 28 of the die 16, the volume within the tubular interior plus the increasing volume of the pressure chambers 50 is such as to approach that volume which has been displaced by the plunger 76, resulting in a decrease in pressure within the tubular blank. At this point, the plunger 76 is retracted from the tubular interior, causing a decrease in the pressure therein, wherein the fluid 14 within the tank 12 will be dawn in past the check valve 72, through the axial passage 78 and into the interior of the tubular blank 25. At this point, the plunger 76 may be again extended inwardly into the tubular blank interior further displacing the fluid therein and creating another pressure increase in the same manner as hereinbefore described. This process of extending and retracting the plunger 76 may be repeated until the tubular blank 25 has been expanded to completely conform with the contour of the die halves 18, 20.

Upon completion of the forming stage, the blocks 36,

38 and their associated plugs 32, 34 are withdrawn from the tubular blank 25 and the opposing halves of the die 16 are retracted to the position shown in FIGS. 1 and 5. The part ejector 29 illustrated in FIG. 5 moves vertically upward as hereinbefore described to eject the formed tubular blank 25 from between the opposing halves of the die 16. A new tubular blank 25 is then inserted vertically downward between the die halves and the forming process is commenced in the manner as hereinbefore described. It can be seen that the present invention is adaptable to automatic handling for high volume production.

In certain applications and particularly in the forming of tubular blanks made of a relatively soft metal, it may be desirable to maintain a fluid sealing relationship between the plugs and the engaged portion of the tubular blank 25 without permitting axial inwardly movement of the outer ends of the tubular blank 25.

Such an arrangement is illustrated in FIG. wherein a modified plug 100 is inserted within the tubular blank 25 and is sized in the same manner as described hereinbefore in description of the plugs 32, 34; that is, the outer diameter of the step section 101 exceeds the inner diameter of the tubular blank 25 by a predetermined amount. The step section 101 is provided with a generally frusto-conically shaped annular land 102 which engages the end portion 104 of the tubular blank interior wall to expand the same in a generally flared manner as illustrated in FIG. 10. Mating annular lands 106 formed on the outer ends of the die halves 18, are adapted to receive the outer surface 108 of the flared end portion 104 forming steps which prevent axial inwardly movement of the tubular blank as the same is formed by the application of pressure therein in the same manner as hereinbefore described. As the pressure within the tubular blank 25 is communicated to the pressure chambers 50 and the force acting on the areas 48 urges the plug 100 inwardly, the annular lands 102 engage the inner surfaces 110 of the flared sections 104 with a greater force thus ensuring a fluid tight seal which increases as the pressure within the tubular interior increases, while at the same time the outer surfaces 108 of the flared sections 104 are urged into abutment with the annular lands 106 formed on the opposite ends of the die 16. The abutment between the outer surfaces 108 of the flared sections 104 and the annular lands 106 formed on the die prevents any inward axial movement of the tubular blank ends.

In view of the above, it can be seen that the several objects of the invention are achieved and other advantageous results attained.

Having thus described the invention, what is claimed is as follows:

1. An apparatus for forming a tubular blank comprising:

a forming die having a longitudinal bore for receiving and forming said tubular blank;

a pair of plugs axially aligned with said bore and movably positioned to enter and engage the interior of said tubular blank from opposite ends thereof such that the outer surfaces of said plugs and the inner surfaces of said opposite ends each engage along a predetermined distance, said plugs being of a shape conforming to the shape of the openings of said blank interior and being of a greater size than said openings so as to create an interference between the engaging surfaces of said plug and said blank interior whereby said blank is expanded outwardly along said predetermined distance thereby creating a fluid sealing engagement between said engaging surfaces, at least one of said plugs having a passage extending to the inner face thereof which enters said tubular blank interior;

means for supplying a fluid to the interior of said tubular blank;

means associated with said last mentioned passage for pressurizing said fluid; and

the blank engaging surface of each plug having an end portion substantially uniformly sized in crosssect-ion throughout its length and an outwardly flaring portion adjacent said end portion for engaging an end portion of said blank, and said forming die having an interior end surface shaped complementary to said end and flaring portion but radially spaced therefrom substantially equal to the thickness of the matenial of the blank, whereby to retain the blank end portion in position between the plug and die against axial displacement during forming.

2. In combination with a tubular blank forming apparatus, comprising:

a forming die having a longitudinal bore for receiving and forming said tubular blank;

a pair of plugs axially aligned with said bore and movably positioned to enter the interior of said tubular blank from opposite ends thereof such that the outer surfaces of said plugs and the inner surfaces of said opposite ends each initially sealingly engage along a predetermined distance, at least one of said plugs having a passage extending to the inner face thereof which enters said tubular blank interior;

means for supplying a fluid to the interior of said tubular blank; and

means associated with said passage for pressurizing said fluid;

the improvements which comprises:

sealing means radially disposed between each plug and the interior wall of said tubular blank at a point along said predetermined distance, said sealing means being responsive to pressure in said tubular blank interior for creating a fluid sealing engagement between said plugs and said tubular blank interior;

each of said sealing means comprising an endless elastomeric material disposed Within a peripheral groove formed around said plug portion engaging the interior of said blank and being sized for reception completely within said groove and inwardly of the periphery of said plug to normally be free of interference with the blank when said fluid is not pressurized; and

a passageway connecting said plug grooves to the interior of said tubular blank whereby pressure in said tubular blank whereby pressure in said tubular blank interior is communicated to the underside of said material for creating a force expanding said material into a fluid sealing engagement with said plug and said tubular blank engaged portion.

3. In combination with a tubular blank forming apparatus comprising:

a forming die having a longitudinal bore for receiving and forming said tubular blank;

a pair of plugs axially aligned with said bore and movably positioned to enter the interior of said tubular blank from opposite ends thereof such that the outer surfaces of said plugs and the inner surfaces of said opposite ends each engage along a predetermined distance, at least one of said plugs having a passage extending to the inner face thereof which enters the tubular blank interior;

means for supplying a fluid to the interior of said tubular blank; and

means associated with said passage for pressurizing said fluid;

the improvement which comprises:

piston means associated with each of said plugs,

said piston means being responsive to the pressure in said tubular interior to urge said plugs inwardly into said die longitudinal bore;

9 10 and passage means connecting said piston means 2,203,868 6/1940, Gray et a1. 7258 with said tubular interior. 2,833,330 5/1958 Love 7259 4. The combination as described in claim 2, wherein 3,350,905 11/1967 Ogura et a1. 1 7262 the width of said peripheral groove is substantially equal 3,416,343 12/1968 Thielsch 7258 to the width of said elastomeric material. 5

5. The combination as described in claim 5, including REIG PATENTS a step section formed on each of said plugs, said step 732 953 6/1955 Great Britain 7 5 sections abutting the ends of said tubular blank. 1,103,717 1955 France 72 0 R f r Cited 10 RICHARD J. HBRBST, Primary Examiner UNITED STATES PATENTS 788,119 4/1905 Pope 7258 72 62 1,886,831 11/1932 Murray 72 58 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 625, 040 Dated December 7, 1971 lnventofls) William J. DeGain (KTD-ll3-A) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE SPECIFICATION:

Col. 3, Line 19, change "from" to -form- Col. 6, line 54, change "5" to -25- IN THE CLAIMS:

Col. 9, line 6, change "5" to 3- Signed and sealed this 13th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attasting Officer Commissioner of Patents 

